EP3720815A1

EP3720815A1 - Method for providing co2 for synthesizing urea from flue gas and syngas

Info

Publication number: EP3720815A1
Application number: EP18812145.3A
Authority: EP
Inventors: Yevgeny Makhynya; Tarek EL-HAWARY
Original assignee: ThyssenKrupp AG; ThyssenKrupp Industrial Solutions AG
Current assignee: ThyssenKrupp AG; ThyssenKrupp Industrial Solutions AG
Priority date: 2017-12-06
Filing date: 2018-11-30
Publication date: 2020-10-14
Anticipated expiration: 2038-11-30
Also published as: DE102017222030A1; WO2019110443A1; DK3720815T3; EP3720815B1

Abstract

The invention relates to a method for providing carbon dioxide for synthesizing urea from ammonia and carbon dioxide, wherein a carbon dioxide-containing flue gas is used as a first starting gas mixture. The first starting gas mixture is supplied to a low-pressure absorber (12), in which carbon dioxide from the first starting gas mixture is absorbed in a liquid solvent based on an ammonia/water mixture. Furthermore, a carbon dioxide-containing syngas is used as a second starting gas mixture, and the second starting gas mixture is supplied to a high-pressure absorber (15), in which carbon dioxide from the second starting gas mixture is absorbed in a liquid solvent at an increased pressure. According to the invention, for the absorption step in the low-pressure absorber (12) and the absorption step in the high-pressure absorber (15), the same carbon dioxide-depleted liquid solvent is used, which is supplied to the high-pressure absorber and the low-pressure absorber in at least two parallel sub-flows. Preferably, a first carbon dioxide-enriched solvent flow (16) exiting the low-pressure absorber (12) and a second carbon dioxide-enriched solvent flow (19) exiting the high-pressure absorber (15) are then combined in order to form a common solvent flow (27). The method allows carbon dioxide to be provided for synthesizing urea from ammonia and carbon dioxide, said method having a lower specific energy consumption and reduced capital costs compared to known methods.

Description

Method of providing CO _? for the synthesis of urea from smoke and synthesis qas

The present invention relates to a method for providing carbon dioxide at high pressure and temperature level for the synthesis of urea from ammonia and carbon dioxide, in which a carbon dioxide-containing flue gas is used as a first starting gas mixture, said first starting gas mixture is fed to a low-pressure absorber, in the carbon dioxide from the first starting gas mixture is absorbed in a liquid solvent based on an ammonia-water mixture, wherein further used as a second starting gas mixture, a carbon dioxide-containing synthesis gas and this second starting gas mixture is fed to a high-pressure absorber, in the carbon dioxide from this second starting gas mixture in a liquid solvent based on an ammonia-water mixture is absorbed at elevated pressure.

The large-scale production of urea is currently based almost exclusively on the high-pressure synthesis of ammonia and carbon dioxide in a urea plant at a pressure of about 150 bar and a temperature of about 180 ° C.

Both starting materials for the urea synthesis are usually provided in an ammonia plant, which is usually in direct proximity to the respective urea plant. Since the ammonia is usually liquid at the plant boundary of the ammonia plant, it can be brought to the pressure level of the urea synthesis with a limited energy and equipment expense. However, the carbon dioxide falls in the ammonia plant gaseous and it is therefore a much greater energy and equipment required to bring the carbon dioxide to the pressure level of the urea plant.

As a rule, a conventional ammonia plant produces about 10% more ammonia than would be necessary for a stoichiometric conversion of the ammonia with the carbon dioxide to urea. If, on the other hand, all of the ammonia produced and the stoichiometric carbon dioxide required for this purpose are converted to urea, this is referred to as a so-called "balanced plant". For the construction of such a "balanced plant" further equipment and procedural measures are required in comparison to the classic ammonia-urea plant complex.

Renewal and modernization of existing plants (so-called "revamp") as well as the construction of "balanced plant" plants often require a source of additional carbon dioxide. This carbon dioxide is available for example from the flue gas. However, flue gases fall at atmospheric pressure and there separated carbon dioxide must be energetically brought to the pressure of the urea synthesis. The equipment required for this is very high, especially when large additional amounts of carbon dioxide are needed. In the case of revamping on an existing plant, the most critical plant areas are usually the C0 ₂ scrubber and the C0 ₂ compressor. The carbon dioxide required for the urea synthesis is produced in the synthesis gas production of an ammonia plant as a constituent of the crude synthesis gas after reforming. Since the carbon dioxide would act as a catalyst poison in the ammonia synthesis, it must be separated from the synthesis gas. In the prior art, this is usually done by using regenerative gas scrubbing, with a larger number of selectively acting solvents are known that are useful for this gas scrubbing.

It is also known to separate carbon dioxide from flue gases by regenerative gas scrubbing with selectively acting solvents or with ammonia or ammonia-water mixtures. In the latter case, the carbon dioxide in the solution is bound mainly chemically in the form of carbamate and carbonate ions. The loaded solution is thermally regenerated, the carbon dioxide is produced without pressure or almost without pressure. However, detailed investigations by the applicant have shown that carrying out gas scrubbing with an ammonia-water mixture under these conditions has no energy advantages over washes with selective solvents such as alkanolamines.

From DE 10 2015 121 756 Al a process for the production of urea is known in which a gas stream comprising hydrogen, nitrogen and carbon dioxide is provided, at least part of the carbon dioxide is separated from this gas stream with a solvent, ammonia from at least part of the Hydrogen, and at least a portion of the nitrogen contained in the carbon dioxide-depleted gas stream is synthesized, the carbon dioxide is desorbed from the solvent, and urea is produced from the previously synthesized ammonia and the previously desorbed carbon dioxide. In this known method, the starting gas mixture containing hydrogen, nitrogen and carbon dioxide is a synthesis gas which is produced by steam reforming or by autothermal reforming. The separation of the carbon dioxide is carried out in this process by an ammonia-water scrubbing. The carbon dioxide-enriched solvent leaving the separator is compressed to a target pressure above the urea synthesis pressure and the compressed solvent is fed to a desorption apparatus in which the carbon dioxide is desorbed from the solvent stream at a pressure which is above the operating pressure of the urea synthesis unit. The desorbed in the desorption carbon dioxide is passed into the urea synthesis unit and reacted there with the ammonia to form urea.

In the US 9,428,449 B2 an integrated ammonia and urea synthesis is described in which a synthesis gas generated in a reformer is first subjected to a CO shift reaction and the resulting mixture of hydrogen and carbon dioxide is fed to a high-pressure absorber, in which a gas scrubber with a medium greasy solvent flow takes place. In the reformer, a flue gas stream is also produced, which is fed to a low-pressure absorber, in which a gas scrubbing with a depleted (lean) solvent takes place, whereby a nitrogen-containing stream is obtained, which is fed to an ammonia synthesis reactor. By the gas wash of the Synthesis gas in the high pressure absorber, the carbon dioxide is removed from this gas stream and obtained a hydrogen-containing stream, which is also fed to the ammonia synthesis reactor, wherein the synthesized there ammonia is then fed to a urea synthesis reactor. The term "integrated ammonia and urea synthesis" can be used for this known process because the ammonia produced in the plant complex is used immediately for the CO ₂ separation, the reaction product ammonium carbamate is an intermediate of the urea synthesis, and this intermediate is immediately available for the Urea synthesis is used. In this known method is used as a lean solvent, a recycle stream from the urea plant containing ammonium carbamate, ammonium carbonate, ammonium bicarbonate in solution or as a slurry, as well as unreacted ammonia, carbon dioxide and water, this stream optionally further ammonia is added. This lean solvent is then first fed to the low pressure absorber where carbon dioxide is absorbed from the flue gas to yield a carbon dioxide enriched solvent, which is then fed to the high pressure absorber where further carbon dioxide from the synthesis gas is absorbed. The highly enriched solvent from the high-pressure absorber, which contains ammonia and water in addition to carbon dioxide, is then fed to the urea synthesis. The carbon dioxide depleted solvent from the urea synthesis reactor is then passed again as a lean solvent to the low pressure absorber.

It is typical of an integrated synthesis, as described in US Pat. No. 9,428,449 B2, in which an ammonia synthesis reactor and a urea synthesis reactor are integrated into a plant concept that C0 ₂ -containing gases with ammonia or with very concentrated ammonia solution in water in Be brought in contact. In the absence of water, predominantly ammonium carbamate is formed, which is passed on directly into the urea synthesis. It is obvious that this can only work with very small amounts of water in the wash solution since the water is undesirable for both carbamate and urea synthesis. Another peculiarity of integrated processes of this kind, as known from the prior art, is the fact that both parts of the plant (for ammonia and urea synthesis) can only be operated together, which makes the operation inflexible and the startup ups and shutdowns of plant components considerably more difficult. For this reason probably no integrated plant in industrial size was built yet.

The object of the present invention is to provide a method for providing carbon dioxide for the synthesis of urea from ammonia and carbon dioxide having the features of the aforementioned type, which has a lower specific energy consumption and reduced capital costs compared to known methods.

The solution of the above object provides a method of the type mentioned above with the features of claim 1. According to the invention, it is provided that the same carbon dioxide-depleted liquid solvent is used for the absorption step in the low-pressure absorber and for the absorption step in the high-pressure absorber, which is supplied to the high-pressure absorber and the low-pressure absorber in at least two parallel partial streams.

Unlike the method known from US Pat. No. 9,428,449, according to the invention, the washing process for removing the carbon dioxide from the gas stream is not successive so that an already partially carbon dioxide-enriched solution passes from the low-pressure absorber into the high-pressure absorber, but the washing process takes place in parallel with the high-pressure absorber similar to carbon dioxide depleted washing solution, which is divided into two sub-streams, one of which is fed to the low-pressure absorber and the other is supplied to the high-pressure absorber. The expression "same wash solution depleted in carbon dioxide" preferably also encompasses, within the meaning of the invention, a slight deviation in the composition of the two partial flows. Physical processes, such as degassing dissolved gases, may require depressurization. It is preferable to use the same carbon dioxide-depleted wash solution, which may have slight variations in composition and temperature. These deviations are preferably due to the further inevitable relaxation (and thus energy recovery) due to pressure level of the low-pressure absorber. Preferably, at a pressure release (for example from 36 bar to 1 bar), the main components C0 ₂ , NH ₃ and H ₂ 0 remain virtually unchanged (+/- 10%). Only small amounts of physically dissolved gases, especially H ₂ and N ₂ , escape. This has the advantage that a depleted wash solution instead of a partially loaded with carbon dioxide solution enters the high-pressure absorber.

According to a preferred development of the process according to the invention, the first carbon dioxide-enriched solvent stream leaving the low-pressure absorber and the second carbon dioxide-enriched solvent stream leaving the high-pressure absorber are optionally combined after the removal of inert gas to form a common solvent stream after the removal of inert gas.

Unlike the prior art, however, this enriched with carbon dioxide solvent stream from the two absorbers is not fed to the urea plant, but the solution is brought to high pressure and the carbon dioxide is expelled from the solvent. Preferably, the carbon dioxide is then desorbed either from a first, from the low-pressure absorber exiting with carbon dioxide-enriched solvent stream or from a second, emerging from the high-pressure absorber with carbon dioxide-enriched solvent stream or from a combined common solvent stream of low-pressure absorber and high-pressure absorber in a desorption.

Applicant's investigations have shown that the ammonia-water scrubbing of flue gas and syngas in the two absorbers can be operated to provide both an improvement specific energy consumption and a reduction in the cost of capital. These advantages are achieved in particular by the carbon dioxide is not expelled from the solvent as usual at a relatively low pressure, but at a pressure level preferably slightly above the urea synthesis pressure, so that the desorbed carbon dioxide can then be easily introduced into the urea synthesis and without further compression work. Unlike in the prior art, therefore, not loaded with carbon dioxide washing solution (rieh solvent) of the urea synthesis is supplied, but the desorbed from the wash solution carbon dioxide. The depleted solution after the desorption process can preferably be returned as a washing solution to the two absorbers and thus reused in the absorption process to remove the carbon dioxide. For example, a C0 ₂ compressor is no longer necessary.

According to a preferred embodiment of the method, the carbon dioxide-enriched solvent is desorbed in the desorption at elevated temperature from the solvent, preferably at a temperature of at least 150 ° C, more preferably at a temperature of at least 180 ° C, for example at about 200 ° C. , A significant advantage of carbon dioxide scrubbing, as practiced in the process of the present invention, is that the carbon dioxide is inevitably provided at high pressure and via thermal regeneration in the desorber at a relatively high temperature. Through these measures, carbon dioxide can be provided at a temperature and pressure level which is optimal for urea synthesis and does not require any further process engineering steps for the adaptation of pressure and temperature.

Only one needs a slightly higher pressure to overcome the pressure losses on the way to the urea plant. It is known from thermodynamics that in distillation in ternary systems, the pressure increase actually causes the most volatile component to be "squeezed" out of the solution, thus increasing the partial pressure of the lightest component over the mixture. In the ternary mixture NH ₃ -CO ₂ -H ₂ O, the C0 _{2 is} the most volatile component. Therefore, an increase in pressure in the method according to the invention after absorption and relaxation is particularly advantageous.

It is particularly advantageous if this pressure increase is carried out in accordance with a preferred embodiment of the method according to the invention by means of a pump in the liquid state, thereby saving a C0 ₂ compressor and the associated periphery. This measure reduces both the energy consumption costs and the capital costs of the plant.

Preferably, the procedure is followed by first cooling the carbon dioxide-enriched solvent from the high-pressure absorber and then releasing it, wherein gases absorbed in the solvent which are different from carbon dioxide (inert), in particular nitrogen and / or hydrogen, are gassed out of the solvent and be separated. Subsequently, one combines the carbon dioxide-enriched solvent from the high-pressure absorber with the enriched Solvent from the low pressure absorber and preferably thereafter increases the pressure of the combined solvent stream before feeding it to the desorption device.

Studies in the context of the present invention have shown that the carbon dioxide with the required purity can be extracted from the solvent at a still moderate temperature level. However, the required temperature level is still high, for example at about 200 ° C, so that conventional selective solvents which are otherwise used in gas scrubbing, such as alkanolamines, can not be used because of their limited thermal stability. Because of the relatively high vapor pressure even at moderate temperatures and the relatively large Desorptionswärme ammonia comes as a pure solvent for gas scrubbing out of the question. Also you can not regenerate pure ammonia as a detergent and recycle, so that the ammonia would have to be consumed. In this case, ammonia would lead to carbamate, which would then have to be fed into urea synthesis. This would in principle again result in an "integrated" method which the present invention wishes to avoid.

Therefore, it is proposed in the process according to the invention to use a dilute ammonia solution for the CO ₂ scrubbing based on ammonia-water in the two absorbers, in particular an aqueous ammonia solution having a concentration of preferably about 15-25% by weight. The expelled in the desorption carbon dioxide can be provided for the synthesis of urea. The inventive process develops its full potential when used in an ammonia-urea complex. But it is also possible to use the inventive method in a stand-alone ammonia plant. This shows the high flexibility of the process.

According to the present invention, the gas scrubbing is carried out in both absorbers with the same ammonia-water solution, on the one hand to bind carbon dioxide from syngas and on the other hand carbon dioxide from flue gas. According to a preferred variant of the method, an apparatus is used which can be subdivided into, for example, two zones (which is not mandatory), namely a high-pressure absorber and an absorber operating at atmospheric pressure. Consequently, the carbon dioxide-laden solution from the high-pressure absorber must subsequently be expanded in order to bring it to a pressure of, for example, about 2 bar, wherein gases which are also absorbed for the synthesis of urea, in particular hydrogen and nitrogen, are also released in the washing solution. The loaded with carbon dioxide washing solution from the low-pressure absorber (atmospheric absorber), however, must be brought to a slightly higher pressure, for example, 2 bar and then you can unite the two streams.

Subsequently, the combined washing solution is preferably brought by means of one or more pumps to a pressure which is slightly above the urea synthesis pressure and the washing solution is thermally regenerated in the desorption. Preferably, carbon dioxide depleted solvent from the desorbing apparatus is at least partially recycled to the low pressure absorber and / or at least partially to the high pressure absorber.

According to a further development of the method, carbon dioxide-depleted solvent from the desorption device is first depressurized in at least a first turbine using the stored energy to a lower pressure and a partial flow of this solvent is returned to the high-pressure absorber and a further partial stream of this solvent is formed relaxed in at least a second turbine using the stored energy to an even lower pressure and then at least partially returned to the low-pressure absorber.

Furthermore, it may be advantageous if the combined solvent stream from the low-pressure absorber and the high-pressure absorber, before it is fed to the desorption, is divided into at least two partial streams for the purpose of heat integration, of which the first partial flow is fed to a first heat exchanger and the second partial flow of a second Heat exchanger is supplied, wherein after flowing through the two heat exchangers, the two partial streams are reunited and then fed to the desorption.

A preferred embodiment of the method provides for a treatment of the flue gas stream before it is supplied to the low-pressure absorber. For this purpose, for example, the flue gas can be heated or cooled before the step of absorption in the low-pressure absorber. Or the flue gas is filtered or wet scrubbed prior to the step of absorbing in the low pressure absorber to remove fly ash or soot particles.

The inventive method is applicable both in the construction of new ammonia-urea complexes as so-called "balanced plant" as well as the capacity expansion of an existing urea plant or capacity expansion of an existing ammonia-urea complex in the event of a slight increase in capacity of the ammonia Plant or significant capacity expansion of the urea plant, as well as in the case of significant capacity expansion of ammonia and urea plants. In this context, a slight increase in the capacity is understood to mean an expansion of, for example, about 5% to about 10%, and a significant increase in the capacity is understood as meaning, for example, about 10% to about 30% or more. In cases of capacity expansion, it is advisable to operate the ammonia-water scrubbing in the two absorbers in parallel with a scrub already in the system, irrespective of the type of scrubbing.

The amount of heat required for the desorption of the carbon dioxide can be made available in both the above-mentioned variants new plant or capacity expansion with limited modifications of the steam system in an ammonia-urea complex. As the steam demand for the Drive the C0 ₂ -Verdichters deleted, resulting in particular in a new system, a significant reduction in energy consumption, which can be reflected for example in a reduced firing capacity of an existing auxiliary boiler. In the case of a capacity expansion, the energy savings can be correspondingly lower. However, the C0 ₂ -Verdichter is not additionally claimed, or for larger capacity increases no additional C0 ₂ -evaporator is necessary. With the method according to the invention, virtually any additional amount of C0 ₂ can be made available for a urea synthesis.

The greatest reduction in capital investment costs is to be expected in the case of a new plant, as the elimination of the C0 ₂ compressor including its ancillary equipment such as propulsion turbine, intercooler, oil system, etc. and the synergistic use of the high pressure desorber for the wash solutions from both absorbers to a substantial Savings result.

The inventive method also allows separate operation of an ammonia plant without operation of the urea plant, since the carbon dioxide can then be discharged as in the conventional process management to the environment. In this case you do not need to operate the atmospheric absorber.

The present invention furthermore relates to a plant for the provision of carbon dioxide for the synthesis of urea from ammonia and carbon dioxide, comprising at least one low-pressure absorber, a device for supplying a first starting gas mixture comprising a flue gas containing carbon dioxide to this low-pressure absorber, a device for supplying a liquid Solvent based on an ammonia-water mixture to this low-pressure absorber, a high-pressure absorber, a device for supplying a second starting gas mixture comprising a carbon dioxide-containing synthesis gas to this high-pressure absorber, a device for supplying a liquid solvent based on an ammonia-water mixture to this High-pressure absorber, at least one output line to the low-pressure absorber for discharging carbon dioxide-laden solvent from the low-pressure absorber, at least one output line to the Hochdruckabsabs The invention further provides at least one desorption device suitable for expelling carbon dioxide at elevated pressure from the carbon dioxide-laden solvent.

According to a preferred development of the invention, the low-pressure absorber and the high-pressure absorber are arranged in a common apparatus, which comprises a low-pressure zone and a high-pressure zone. In this variant, the carbon dioxide wash of both the flue gas and that of the synthesis gas can be carried out in a common apparatus with the same washing solution. Preferably, the plant comprises at least one first means disposed in the conduit path of a conduit for depleted solvent drained from the desorption apparatus between the desorption apparatus and the high pressure absorber to depressurize the depleted solvent. After expansion, for example in a turbine, the depleted solvent can be recycled to the high pressure absorber.

Preferably, the plant according to the invention further comprises at least one second turbine, which is arranged in the line path of a line for depleted from the desorption depleted solvent between the desorption and the low pressure absorber and downstream of the first turbine to further relax the depleted solvent, so that the pressure so far is lowered, that the depleted solvent for the renewed gas scrubbing can be returned to the low pressure absorber.

According to a preferred development of the invention, the outlet line of the low-pressure absorber and the outlet line of the high-pressure absorber open into a common line for carbon dioxide-laden solvent, which leads to the desorption device. In this way it is possible to combine the solvent streams of the carbon dioxide-laden wash solutions from both absorbers and together supply the desorption device to drive off the carbon dioxide.

Preferably, at least one means, preferably at least one pump, is provided in the conduit path from the low pressure absorber to the desorption apparatus to increase the pressure of the loaded solvent since the carbon dioxide is preferably expelled at an elevated pressure above the pressure of the urea synthesis.

Furthermore, preferably in the conduction path from the high-pressure absorber to the desorption at least one device, preferably a turbine arranged to reduce the pressure of the loaded solvent, so that this stream of loaded solvent is freed from inert and is brought to about the same pressure as that from the low-pressure absorber dissipated solvent stream and then both loaded solvent can be combined into a common stream, which is fed to the desorption.

Preferably, further downstream of the junction of the outlet line of the low-pressure absorber in the output line of the high-pressure absorber in the common line to the desorption at least one device, preferably a pump is arranged to increase the pressure of the loaded solvent. The two solvent streams of the loaded solvent from the two absorbers can then be combined first at approximately the same pressure and then the pressure is further increased before the combined solvent stream is fed to the desorption device.

Hereinafter, the present invention will be described with reference to an embodiment with reference to the accompanying drawings. Showing: Figure 1 is an exemplary plant schematic of a plant for the provision of carbon dioxide for the synthesis of flores.

In the system according to the invention, two starting gas streams are provided, each of which contains carbon dioxide, namely a flue gas stream, which flows via an input line 10 through a heat exchanger 11 and a working at atmospheric pressure low pressure absorber 12 is supplied. In this low-pressure absorber 12 is a gas scrubbing by means of a dilute aqueous ammonia solution, so u.a. To absorb carbon dioxide from the flue gas stream.

The second starting gas stream is a synthesis gas stream, which flows via the inlet line 13 through a heat exchanger 14 and is then fed to a liquid pressure absorber 15, in which also a gas scrubbing with a dilute aqueous ammonia solution takes place to remove carbon dioxide from the synthesis gas. A feature of the present invention is that two different sources are used for the provision of carbon dioxide, wherein in both absorbers, the gas scrubbing can be done with the same scrubbing solution.

After gas scrubbing in the low pressure absorber 12, a carbon dioxide-enriched solvent (rieh solvent) exits the low pressure absorber via line 16, passes through a heat exchanger 17, and is then pumped from about atmospheric pressure in the low pressure absorber to an elevated pressure of, for example, about 2 brought bar, so that this solution of the loaded solvent can later be combined with the corresponding enriched wash solution from the Flochdruckabsorber before the pump 26. The carbon dioxide-enriched solvent leaves the flohdruckabsorber 15 via the line 19, then flows through a heat exchanger 20, in which the heat integration takes place, is expanded, for example via a turbine 21 and is fed via the line 22 to a separator 23. By lowering the pressure, inert gases such as, for example, hydrogen and nitrogen can be outgassed from the solvent stream in the separator 23 for the synthesis of the flarnane and removed via the line 24. The pressure reduction of the solvent flow from the flohdruckabsorber 15 in the turbine 21 can be carried out to a pressure of for example about 2 bar, so that this pressure is about the same as that of the solvent flow from the low-pressure absorber 12. Primarily, this pressure reduction to a certain pressure serves the purpose of outgassing the inert (mainly hydrogen and nitrogen) and maximally retaining the other components of the mixture, especially ammonia and carbon dioxide. This target pressure does not necessarily correspond approximately to the pressure in the low-pressure absorber. At the branch 24 a then opens the line 16 from the low-pressure absorber 12 in the line 25 from the separator 23, so that the two solvent streams of the loaded solvent can be combined from two absorbers and then brought by the pump 26 to an elevated pressure. The combined stream of the laden solvent then flows through line 27 through the Heat exchanger 20, where a heat exchange with the medium takes place in the line 19 coming from the high-pressure absorber 15.

The loaded solvent brought to an elevated pressure then flows through a further heat exchanger 28 and is then divided into two partial streams, wherein a first partial stream is passed via the line 29 through a heat exchanger 30 and then fed to a desorption device 31, while a second partial flow over the from the line 29 branching branch line 32 flows, then flows through another heat exchanger 33 and then merged again with the first partial flow and the desorption device 31 is supplied. By this division into the two partial streams, the heat energy contained in the product streams 34 and 38 of the desorption device 31 can be better utilized.

In the desorption apparatus 31, the carbon dioxide contained in the solvent stream is desorbed at an elevated pressure so that carbon dioxide is at a pressure higher than the synthesis pressure of the urea synthesis. The desorbed carbon dioxide is discharged at the top of the desorption device 31 via the line 34, then passed through the heat exchanger 30 and enters a separator 35, in which the condensate, which is formed during the cooling in 30, separated, while the carbon dioxide, which continue Contain ammonia and water can be dissipated via the outgoing from the top of the separator 35 line 36 and fed to a urea synthesis plant.

A separated in the separator 35 stream can flow through the conduit 37 through the heat exchanger 28 and then continue to flow into the conduit 39. The depleted of carbon dioxide solvent stream (lean solvent) leaves the desorption device 31 in the sump region and is guided via the line 38 through the heat exchanger 33 and then merged with the stream from the line 37. This combined stream of depleted solvent then flows via line 39 through a turbine 40 and is thus expanded to a lower pressure using the energy contained in the stream and is then recycled via line 41 as a depleted solvent to the two absorbers. The depleted solvent now has a suitable pressure, so that a partial stream can be branched off from the line 41 and fed to the high-pressure absorber 15 via the line 42 as a washing solution. The remaining partial flow of the depleted solvent initially flows through the line 41 through a further turbine 43 and is relaxed there to a lower pressure, then flows through a heat exchanger 44 and is fed to a separator 45, in which a separation of gases, wherein subsequently this partial stream of the depleted solvent can be supplied via line 46 to the low-pressure absorber 12. In the separator 45 separated gases can be supplied via the emanating from the head region of the separator 45 line 47 of the input line 10 for the flue gas, so that a return of these separated gases in the gas scrubbing can be done together with the freshly supplied flue gas. Alternatively, the separated gases in the separator 45 via the line 47 are fed directly to the atmospheric absorber. After gas scrubbing, a purified flue gas can be removed via the outlet line 48 from the top region of the low-pressure absorber 12.

After cleaning the gas, the synthesis gas purified in the high-pressure absorber 15 can be fed via line 49 to a further separator 50, wherein a purified synthesis gas stream can be removed from the system via the outlet line 51 issuing from the head region of the separator 50. In the bottom region of the additional separator 50, after separation of the purified synthesis gas, a material stream is obtained, which is fed via the line 52 to the separator 23 and thus combined with the stream of laden solvent and further treated in the manner described above and the desorption 31 can be supplied ,

Low-pressure absorber 12 and high-pressure absorber 15 can be housed in a single apparatus, as indicated in FIG. 1, which has two zones, namely a low-pressure zone and a high-pressure zone.

A significant advantage of the method according to the invention results from the fact that after the gas scrubbing in the two absorbers with carbon dioxide enriched solvent is not supplied as such the urea synthesis, but by means of the desorption 31, the carbon dioxide is expelled from the solvent and suitable for the urea synthesis high Pressure and comparatively high temperature is provided. The lean solvent discharged from the desorption apparatus can be recycled and thus used again for the absorption process in the two absorbers.

LIST OF REFERENCE NUMBERS

10 input line

11 heat exchangers

12 low-pressure absorber

13 input line

14 heat exchangers

15 high pressure absorber

16 line

17 heat exchangers

18 pump

19 line

20 heat exchangers

21 turbine

22 line

23 separators

24 output line

24 a branch office

25 line

26 pump

27 line

28 heat exchangers

29 line

30 heat exchangers

31 desorption device

32 branch line

33 heat exchangers

34 line Separator line

management

Line Turbine Line Line Turbine Heat exchanger Separator Line Return line Output line Line separator Output line Line

Claims

claims

1. A method for providing carbon dioxide for the synthesis of urea from ammonia and carbon dioxide, in which a carbon dioxide-containing flue gas is used as a first starting gas mixture, said first starting gas mixture is a low pressure absorber (12) is supplied in the carbon dioxide from the first starting gas mixture is absorbed in a liquid solvent based on an ammonia-water mixture, wherein furthermore a carbon dioxide-containing synthesis gas is used as a second starting gas mixture and this second starting gas mixture is fed to a high-pressure absorber (15), in the carbon dioxide from said second starting gas mixture in a liquid Solvent based on an ammonia-water mixture is absorbed at elevated pressure, characterized in that for the absorption step in the low pressure absorber (12) and for the absorption step in the high pressure absorber (15) the same carbon dioxide depleted liquid ge solvent is used, which is supplied to the high-pressure absorber and the low-pressure absorber in at least two parallel partial streams.

2. The method according to claim 1, characterized in that a first from the low-pressure absorber (12) exiting carbon dioxide-enriched solvent stream (16) and a second from the high-pressure absorber (15) exiting carbon dioxide-enriched solvent stream (19) then to a common solvent stream ( 27).

3. The method according to claim 1 or 2, characterized in that from a first, from the low-pressure absorber (12) emerging with carbon dioxide-enriched solvent stream (16) or from a second, from the high-pressure absorber (15) emerging with carbon dioxide-enriched solvent stream (19) or from a combined common solvent stream (27) of low-pressure absorber and high-pressure absorber is then desorbed in a desorption device (31) carbon dioxide.

4. The method according to claim 3, characterized in that the carbon dioxide-enriched solvent is compressed to a pressure above the urea synthesis pressure, in a desorption device (31) the carbon dioxide at a pressure which is above the urea synthesis pressure is desorbed from the solvent and then the desorbed carbon dioxide is fed to a urea synthesis unit.

5. The method according to claim 3 or 4, characterized in that the carbon dioxide-enriched solvent in the desorption device (31) at elevated temperature the solvent is desorbed, preferably at a temperature of at least 150 ° C, more preferably at a temperature of at least 180 ° C.

6. The method according to claim 2, characterized in that the enriched with carbon dioxide from the flue gas solvent from the low pressure absorber (12) is first brought to an elevated pressure and then combined with the enriched with carbon dioxide from the synthesis gas solvent from the high pressure absorber (15) becomes.

7. The method according to any one of claims 1 to 5, characterized in that the second from the high-pressure absorber (15) emerging with carbon dioxide-enriched solvent stream is then cooled and then relaxed, wherein in the solvent absorbed gases other than carbon dioxide, in particular nitrogen and / or hydrogen ausgast from the solvent and separated.

8. The method according to any one of claims 2 to 7, characterized in that the combined solvent flow of high pressure absorber (15) and low pressure absorber (12) is brought to a higher pressure after the association, the pressure increase preferably by means of a pump (26) in the liquid Condition occurs.

9. The method according to any one of claims 3 to 8, characterized in that from carbon dioxide depleted solvent from the desorption device (31) at least partially to the low pressure absorber (12) and / or at least partially to the high pressure absorber (15) is returned.

10. The method according to claim 9, characterized in that depleted of carbon dioxide solvent from the desorption device (31) during the return first in at least a first turbine (40) using the stored energy to a lower pressure and a partial stream (42) this relaxed Solvent is returned to the high-pressure absorber (15) and a further part stream (41) of this solvent in at least one second turbine (43) using the stored energy to an even lower pressure relaxed and then at least partially returned to the low-pressure absorber (12).

11. The method according to any one of claims 3 to 10, characterized in that the combined solvent stream from the low-pressure absorber (12) and the high-pressure absorber (15) before it is the desorption (31) is supplied, in at least two partial streams (29, 32) is divided, of which the first partial flow (29) is fed to a first heat exchanger (30) and the second partial flow (32) to a second heat exchanger (33) is, after flowing through the two heat exchangers (30, 33), the two partial flows are reunited and then the desorption device (31) are supplied.

12. The method according to any one of claims 1 to 11, characterized in that as

Solvent for carbon dioxide absorption in the two absorbers (12, 15) a dilute aqueous ammonia solution is used.

13. The method according to claim 12, characterized in that a dilute aqueous ammonia solution having an ammonia content of 5 to 30 wt .-%, preferably 10 to 25 wt .-%, more preferably 15 to 20 wt .-% is used.

14. The method according to any one of claims 1 to 13, characterized in that the

Flue gas is heated or cooled before the step of absorption in the low-pressure absorber (12).

15. The method according to any one of claims 1 to 14, characterized in that the

Flue gas before the step of absorption in the low pressure absorber (12) to remove

Fly ash or soot particles are filtered or wet-washed.

16. The method according to any one of claims 1 to 15, characterized in that from the top of the low-pressure absorber (12) a flue gas purified of carbon dioxide is removed and / or a purified synthesis gas is discharged from the top of the high-pressure absorber (15).

17. An installation for providing carbon dioxide for the synthesis of urea from ammonia and carbon dioxide, comprising at least one low pressure absorber (12), means (10) for supplying a first starting gas mixture comprising a flue gas containing carbon dioxide to said low pressure absorber (12), means (46) for supplying a liquid solvent based on an ammonia-water mixture to said low pressure absorber (12), a high pressure absorber (15), means (13) for supplying a second starting gas mixture comprising a synthesis gas containing carbon dioxide to said high pressure absorber (15 ), means (42) for supplying a liquid solvent based on an ammonia-water mixture to said high-pressure absorber (15), at least one outlet line (16) on the low-pressure absorber (12) for discharging carbon dioxide-laden solvent from the low-pressure absorber , at least one output line (19) on the High-pressure absorber (15) for discharging carbon dioxide-laden solvent from the high-pressure absorber, characterized in that the system further comprises at least one desorption device (31) suitable for expelling carbon dioxide at elevated pressure from the carbon dioxide-laden solvent.

18. Plant according to claim 17, characterized in that it comprises means (47) for supplying a gas mixture from a separator (45) to the low-pressure absorber (12).

19. Plant according to claim 17 or 18, characterized in that the low pressure absorber (12) and the high pressure absorber (15) are arranged in a common apparatus which comprises a low pressure zone and a high pressure zone.

20. Plant according to one of claims 17 to 19, characterized in that it comprises at least a first turbine (40) which in the conduction path of a line (39) for from the desorption (31) discharged depleted solvent between the desorption device (31) and the high pressure absorbent (15) is arranged to relax the depleted solvent and recover stored mechanical energy.

21. Plant according to claim 20, characterized in that this one at least a second

Turbine (43), which in the conduction path of a line for out of the

Desorber (31) discharged depleted solvent between the desorption device (31) and the low pressure absorber (12) and downstream of the first turbine (40) is arranged to further relax the depleted solvent.

22. Plant according to one of claims 17 to 21, characterized in that the

Outlet line (16) of the low-pressure absorber (12) and the output line (19) of the high-pressure absorber (15) open into a common line (27) for loaded with carbon dioxide solvent, which leads to the desorption device (31).

23. Plant according to claim 22, characterized in that in the line path from the low pressure absorber (12) to the desorption device (31) at least one device, preferably at least one pump (18) is arranged to increase the pressure of the loaded solvent.

24. Plant according to one of claims 22 or 23, characterized in that in

Conduit from the high pressure absorber (15) to the desorption device (31) at least one device, preferably a turbine (21) is arranged to reduce the pressure of the loaded solvent.

25. Plant according to one of claims 22 to 24, characterized in that downstream of the junction of the output line (16) of the low-pressure absorber (12) in the output line (19) of the high-pressure absorber (15) in the common line to Desorption (31) at least one device, preferably a pump (26) is arranged to increase the pressure of the loaded solvent.